cmd/go, cmd/cgo: support older versions of gccgo that lack cgo.Incomplete

cmd/go, cmd/cgo: support older versions of gccgo that lack cgo.Incomplete

Test whether gccgo/GoLLVM supports cgo.Incomplete. If it doesn't, use a
local definition rather than importing it.

Roll back 426496, which skipped a gccgo test, as it now works.

For #46731
Fixes #54761

Change-Id: I8bb2ad84c317094495405e178bf5c9694f82af56
Reviewed-on: https://go-review.googlesource.com/c/go/+/446260
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>